The paradox, is basically a theory about time travel.

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The paradox, is basically a theory about time travel. This thread requires the assumption of making time travel possible. Supposedly, you go back in time and kill your grandfather before he gets married. How did you end up there in the first place?
 

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  • #2
tiny-tim
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he obviously wasn't your biological grandfather :wink:
 
  • #3
ZapperZ
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The paradox, is basically a theory about time travel. This thread requires the assumption of making time travel possible. Supposedly, you go back in time and kill your grandfather before he gets married. How did you end up there in the first place?
Please read this:

http://focus.aps.org/story/v27/st5

Zz.
 
  • #4
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Please read this:

http://focus.aps.org/story/v27/st5

Zz.
I know just enought about quantum physics to realize it is over my head.

But I am curious. What if I went back in time and killed my wife's grandfather? Does this theory allow that? Would I then lose all my memories of her?

This is not a facetious question. Please don't give me a facetious reply.
 
  • #5
ZapperZ
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I know just enought about quantum physics to realize it is over my head.

But I am curious. What if I went back in time and killed my wife's grandfather? Does this theory allow that? Would I then lose all my memories of her?

This is not a facetious question. Please don't give me a facetious reply.
This is why I gave you that link to the Focus article. It tells you, in plain language, that this work has ruled out the quantum version of the grandfather paradox.

Now, you may ask, what does this have anything to do with, say, someone actually being able to do that. The response would be that, if there's a way to construct such "time machine", the simplest and most feasible way, in theory, to accomplish such a thing would be to send such quantum particles. These particles have more abilities to do extraordinary things than common classical objects such as you and me.

Thus, if such paradox can be ruled out at the quantum level, consider such possibility of classical objects that already have bigger problems of even undergoing such time travel.

Zz.
 
  • #6
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The paradox, is basically a theory about time travel. This thread requires the assumption of making time travel possible. Supposedly, you go back in time and kill your grandfather before he gets married. How did you end up there in the first place?
Well, le's assume that alternate realities can exist. Kill grandpa, grandma never gives birth to you, however you continue on your merry way as usual. You're past is from an alternate reality (timeline), so it doesn't matter (far as your own existence goes) that you will never be born in the new reality.

GrayGhost
 
  • #7
JesseM
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This is why I gave you that link to the Focus article. It tells you, in plain language, that this work has ruled out the quantum version of the grandfather paradox.
I think that statement is too strong, the article suggests that the authors have proposed a speculative new principle for how QM would work in a context where time travel was allowed (which doesn't follow directly from the basic laws of QM, since they are formulated in locally flat spacetime where time travel doesn't happen), so that, if the laws of physics respected this principle, paradoxes would be avoided:
Lorenzo Maccone, of the Massachusetts Institute of Technology and the University of Pavia, Italy, and his colleagues propose a more stringent condition that avoids these difficulties. They require that any measurement of the particle going into the future should yield the same result as measuring it when it returns from the past. So any state that would alter the past when it came around again is disallowed, and no grandfather-type paradoxes can arise.
Anyway, it sounds like this is just a new quantum variant of the Novikov self-consistency principle which is how physicists usually propose that paradoxes would be avoided in spacetimes with closed timelike curves (the other way of avoiding paradoxes that GrayGhost mentions, parallel timelines, is not commonly proposed as a resolution although David Deutsch did make a suggestion along these lines)
 
  • #8


The paradox, is basically a theory about time travel. This thread requires the assumption of making time travel possible. Supposedly, you go back in time and kill your grandfather before he gets married. How did you end up there in the first place?
During time travel, assume that two of you are present. The REAL you and the 'you' when you time travel. The time travel 'you' would vanish but the REAL you would lead a forlorn existence as soon as you go back!
 
  • #9
ZapperZ
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During time travel, assume that two of you are present. The REAL you and the 'you' when you time travel. The time travel 'you' would vanish but the REAL you would lead a forlorn existence as soon as you go back!
Why are you quoting yourself while ignoring the responses that you've been given?

Please note that you are now making unsupported speculation, which is in violation of the https://www.physicsforums.com/showthread.php?t=414380" that you had agreed to. Unless this thread is based on either sound physics or peer-reviewed papers, it will be locked.


Zz.
 
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  • #10
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The paradox, is basically a theory about time travel. This thread requires the assumption of making time travel possible. Supposedly, you go back in time and kill your grandfather before he gets married. How did you end up there in the first place?
When you go back in time, you will also grow back in time and thus not exist yet. How can someone who does not exist ever kill someone.
 
  • #11
Fredrik
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When you go back in time, you will also grow back in time and thus not exist yet. How can someone who does not exist ever kill someone.
This is not what anyone means by "go back in time". The general relativity version of time travel is that ordinary motion in some direction in space will somehow take you to an event earlier in the history of e.g. the city you live in. For this to be possible, spacetime must actually contain a curve that describes a way that you can move to accomplish this.
 
  • #12
bcrowell
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  • #13
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I can never understand why people strongly believe in time travels.
Time travels have a big appeal in people's mind, it's a very pervasive "meme".
It's not that time travels are not possible, it's simply they don't make sense.
There's not a "where" to travel to when you would like to go in the past.
There's nowhere to go. Time is not the 4th dimension one can walk up and down like in space.
I cannot understand why people believe it in some way.
Ok, suppose you can go back in time and meet your grandpa.
Now you go back "in your time". I just want to ask: is you grandpa alive even when you are not there with him during your time travels ?
I mean, does your grandpa appears and disappears just when you need it, or is he always alive and living his life completely unaware that he is just a "copy" of another time.
Next, suppose you travel back 50 years to meet your grandpa, I travel back 50 years + 1 day and I meet your grandpa as well.
But the grandpa I meet is not the same that you meet because mine is a day younger.
So how many grandfathers of yours are out there waiting for you ?
Infinite, since time is a continuum there has to be infinite copies of your grandfather waiting for you to go to meet them.
An more, are they just mere copies ? Are they like robot, or zombies ? Or do they have their own personal free will ?
Of course they cannot have free will because it grotesque that each of them can decide how to live their life.
The sentence: "Supposedly, you go back in time and kill your grandfather before he gets married. " has simply no sense.
 
  • #14
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I believe the film "Back to the Future Part II" covered this in detail. You will find your answer there and also punishment for asking, since the movie is horrible.
 
  • #15
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I can never understand why people strongly believe in time travels.
Time travels have a big appeal in people's mind, it's a very pervasive "meme".
It's not that time travels are not possible, it's simply they don't make sense.
There's not a "where" to travel to when you would like to go in the past.
There's nowhere to go. Time is not the 4th dimension one can walk up and down like in space.
I cannot understand why people believe it in some way.
Ok, suppose you can go back in time and meet your grandpa.
Now you go back "in your time". I just want to ask: is you grandpa alive even when you are not there with him during your time travels ?
I mean, does your grandpa appears and disappears just when you need it, or is he always alive and living his life completely unaware that he is just a "copy" of another time.
Next, suppose you travel back 50 years to meet your grandpa, I travel back 50 years + 1 day and I meet your grandpa as well.
But the grandpa I meet is not the same that you meet because mine is a day younger.
So how many grandfathers of yours are out there waiting for you ?
Infinite, since time is a continuum there has to be infinite copies of your grandfather waiting for you to go to meet them.
An more, are they just mere copies ? Are they like robot, or zombies ? Or do they have their own personal free will ?
Of course they cannot have free will because it grotesque that each of them can decide how to live their life.
The sentence: "Supposedly, you go back in time and kill your grandfather before he gets married. " has simply no sense.
I agree with your opening statements, but the post sets time-travel as given, and we're bound by that assumption.
 
  • #16
phinds
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I agree with your opening statements, but the post sets time-travel as given, and we're bound by that assumption.
One should never feel bound by nonsense.
 
  • #17
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Michio Kaku discusses this problem in detail from a "physics perspective" in the "Back to the Future: 25th Anniversary Trilogy". Click on the link below to see what he says.

"Theoretical physicist Michio Kaku talks about time travel in “The Physics of Back to the Future, ” a new bonus feature on the Back to the Future: 25th Anniversary Trilogy, available now on Blu-ray and DVD."


http://www.wired.com/underwire/2010/10/back-to-the-future-physics/
 
  • #18


In effect, you would enter another universe after you went back and killed your grandfather and then returned to the future from that point. The future you returned to would be the one in a universe in which you had never been born. The past (your past) cannot be altered. In your past, your grandfather obviously wasn't killed before he had children, otherwise you would not exist. Intriguingly, if you change your mind afterwards, you can't get back to your original universe as, if once more you go back and stop yourself from performing this heinous act, once you travel forward in time again you enter yet another universe. Why? Because you have no memory of you trying to stop yourself from killing your gramps. What I find interesting is that when you travel back in time you are still travelling forwards in your own time.

So the paradox can be resolved by jumping universes. This idea was played with in the movie, 'Back to the Future', but for the plotline's sake they cheated with it, as, when old Biff stole the time machine and went back to the fifties with the sports almanac and by doing so change the past, he still was able to return with the time machine to the same future that he left, which he wouldn't have been able to do. He would have just disappeared as far as Doc and Marty were concerned, never to return and leaving them stuck there.
 
  • #19
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If we can produce that kind of energy able to transform every particle in the universe, so space and time get altered, it will be possible to travel in time, but aswell you will not be able to have your consciousness and physical body back with you same as it is in the present, but u will change same as univers, I see it more like a video tape that you turn back.
Now, if for a moment we have a technology, that transform everything in the universe, but do not alter your physical body and consciousness, and lets say you get back in the past. Even if everything looks like it is the past it isnt, becouse you arent part of that past, if we look the past from your point of view, where you are now after traveled with this machine it is your present and future.
If you kill, your grandpa, (well it isnt your granpa anymore) you will just be a murderer, nothing else will happen. You will live get old and die, sure if police dont catch you :)

If you don't kill your grandpa, well after few years, you will see yourself born, get old in years etc. But to calculate how the future can be is really hard from my point of view, becouse univers is infite(some say no) and u cant predict how every particle can be.
 
  • #20


I wondered if time travel was simply a reversal of the arrow of time when travelling backwards but surely you would have to reverse with it in that case? Otherwise, if you use a wormhole or other direct form of time travel, you are still travelling forwards in time as far as you're concerned, even if travelling between one side of the wormhole and the other is instantaneous.
 
  • #21
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I wondered if time travel was simply a reversal of the arrow of time when travelling backwards but surely you would have to reverse with it in that case? Otherwise, if you use a wormhole or other direct form of time travel, you are still travelling forwards in time as far as you're concerned, even if travelling between one side of the wormhole and the other is instantaneous.
I don't know much about wormholes. A wormhole is a hypothetical topological feature of spacetime. That would be, fundamentally, a shortcut through spacetime. I see it more like a teleportation, maybe, who knows.
 
  • #22
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One should never feel bound by nonsense.
It is nonsense, as far as I'm concerned, but I don't feel bound by playing the game by the poster's rules. Unless they are assuming topics banned, or should be banned, by PF.
 
  • #23
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I wondered if time travel was simply a reversal of the arrow of time when travelling backwards but surely you would have to reverse with it in that case? Otherwise, if you use a wormhole or other direct form of time travel, you are still travelling forwards in time as far as you're concerned, even if travelling between one side of the wormhole and the other is instantaneous.
It's impossible to discuss this post without examining the reality-architecture of every theoretical view, and then answer for each. 'Results may vary.'

That means reading a lot of posts in PF, or a good book. Almost every content-word and phrase used above carries a whole truckload of assumptions.
 
  • #24
bcrowell
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If we can produce that kind of energy able to transform every particle in the universe, so space and time get altered, it will be possible to travel in time, but aswell you will not be able to have your consciousness and physical body back with you same as it is in the present, but u will change same as univers, I see it more like a video tape that you turn back.
Physics Forums rules, which you agreed to when you registered,
https://www.physicsforums.com/showthread.php?t=414380,
include:
Overly Speculative Posts: One of the main goals of PF is to help students learn the current status of physics as practiced by the scientific community; accordingly, Physicsforums.com strives to maintain high standards of academic integrity. There are many open questions in physics, and we welcome discussion on those subjects provided the discussion remains intellectually sound. It is against our Posting Guidelines to discuss, in most of the PF forums or in blogs, new or non-mainstream theories or ideas that have not been published in professional peer-reviewed journals or are not part of current professional mainstream scientific discussion.
 
  • #25
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I kinda like the way an excerpt from this http://www.webfilesuci.org/wormholeFAQ.html" [Broken]summarizes the way time travel paradoxes are dealt with:

Isn’t time travel by wormhole or any other means impossible due to the paradoxes that it implies?
Not necessarily. Dealing with time travel paradoxes by conjecturing the impossibility of time travel is only one of three ways of resolving the issue. The other ways are:
1) Impose self consistency on classical physics: A time traveler cannot change the past because he was always part of it. When he attempts to change the past, his efforts will be thwarted by an apparent conspiracy of events. 2) Impose self consistency on quantum physics: A time traveler cannot change the past because all possible pasts have already occurred in parallel universes. When he attempts to change the past, his efforts will not seem to him to be thwarted. This is because he will have entered the past of a preexisting parallel universe in which he has already made the changes that he seeks to effect.
 
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